This disclosure relates generally to monitoring of fluid balance status of a subject, and to a mechanism indicative of the fluid balance status of the subject and the form of fluid therapy needed for the subject.
A vast majority of hospitalized patients and all patients undergoing a surgery have an intra-venous (IV) catheter placed to deliver different types of medications for inducing and maintaining anesthesia and/or for infusing fluid volume and blood products, for example. Since the health and safety of the patient is often at stake, an IV infusion system must perform at the highest possible effectiveness, and the patient shall be monitored for early detection of a possible crisis situation. However, current practices in IV therapy seldom include effective crisis management with adequate monitoring and early detection of the effects of the infusions in the patient.
Effective fluid management is based on maintenance of correct fluid balance in human body. In a simple physiological model the water content of the human body is divided into three main fluid compartments: blood, interstitial body fluid (between cells), and intracellular fluid. Usually the intracellular fluid compartment is rather stable, but because of the rapid interchange of fluids that may take place between blood plasma and the interstitial compartment in sepsis and inflammatory states, for example, fluid distribution may change continuously in acute care patients. Further, in operating theatres and during post-operative care, blood and fluid loss may be considerable, which may lead, in an extreme case, to an acute need of blood transfusion and infusion of massive amounts of fluids. Excessive fluid (hypervolemia) may lead to hypertension, heart failure, pulmonary edema or electrolytic imbalance, while too little fluid (hypovolemia) may cause hypotension, hemodynamic collapse, blood centralization, or electrolyte imbalance.
Fluid management may involve infusion of crystalloid solutions, such as physiological saline, colloids or glucose solutions, depending on which compartment of body fluid is most disturbed. Crystalloids fill both the blood plasma and the interstitial compartments, while colloids fill only the plasma compartment. Glucose (small molecule) solutions are used to correct the fluid content of the whole body. In acute care, nutrition and tight glucose control are also beneficial to the patient, and there is also clinical evidence that an effective fluid management can improve patient outcome and reduce hospital costs in acute care.
The clinical signs that are associated with hypo- and hypervolemia, i.e., physiological volemia parameters, are often confounded by various other physiological mechanisms, and are thus unspecific. Still the assessment of volemia parameters requires invasive catheterization, which cannot be justified, if the catheter is not needed for other clinical assessments or IV drug delivery. Further, the control of fluids is currently based on very variable clinical rule sets, since both quantitative measurement of the fluid compartment volumes and the monitoring of the effects of hypo- and hyperfluidity is cumbersome.
Monitoring of blood volume requires that the concentration of a blood substance is measured. Traditionally, the blood volume of a subject has been estimated through an in-vitro analysis of one or more blood samples taken from the subject, by determining the hemoglobin dilution in the samples, i.e., hemoglobin is used as the blood substance whose concentration change is determined before and after diluting the blood with a known amount of fluid.
In order to obviate the continuous blood sampling, it has been suggested to use pulse oximeter technology for measuring the concentration of a tracer substance in blood In a method like this, a known amount of a tracer substance, such as indocyanine green dye, is injected into the subject and the concentration of the substance in the blood is tracked using pulse oximeter technology. The determination of the tracer substance concentration requires that a hemoglobin concentration is determined a priori, as it is used as a reference concentration, to which the measured optical signals are compared. At least one blood sample is thus needed. Based on the tracer concentration, blood volume may be determined, but the measurement is at most semi-continuous, because the dye injection can be repeated only after certain time interval, depending on how fast the dye is eliminated from blood.
However, the hemoglobin concentration measurement or the physiological volemia parameters alone are not specific enough to determine the IV therapy form needed in each individual case. For instance, when the patient looses blood by leaking into the cavities of the abdomen, the hemoglobin concentration does not necessarily change, and hypovolemia can be detected only after the blood pressure compensation mechanisms fail. In this case, the physiological volemia parameters may react to the centralization of blood circulation and may thus reveal the blood leak early enough to start a correcting therapy. The right therapy form is in this case infusion of colloids or crystalloids and, if needed, blood products, to maintain tissue oxygenation and cardiac function. On the other hand, when a part of blood plasma volume runs away to the interstitial fluid compartment, the hemoglobin concentration increases and usually reveals the adverse event and the colloid therapy can be started. The most common fluid administration failure is infusion of too much fluids, which may be caused by human error simply by forgetting, in an emergency situation, to stop rapid fluid expansion. In this case the hemoglobin is diluted and the volemia parameters react simultaneously to the overload of fluid in a patient. Cardiac failure or pulmonary edema can be avoided by stopping infusions and/or medicating the patient as needed.
The drawback of current technology is that it does not allow a comprehensive, easy and quantitative assessment of the fluid balance in a patient. Furthermore, current mechanisms for assessing the fluid balance of a subject are not specific enough for identifying the various reasons behind sudden blood volume changes and thus cannot unambiguously indicate towards the correct form of fluid therapy.